Flat glass, manufactured in a continuous way, is conveyed through a heat insulated tunnel, called a float glass lehr, which is provided with heating means and with cooling means and in which glass is cooled and annealed.
In conventional float glass lehr, the cooling means are composed of a series of exchanging tubes arranged in two horizontal rows inside the tunnel, above and below the glass ribbon. These exchanging tubes are arranged in transverse batteries around in which air flows, initially at the ambient temperature, in the opposite direction to the glass travel direction. The heating means are electrical means and consist of bare or shielded wire heaters located between the exchanging tubes and the glass. The glass cooling can be modified by acting on the air flow rate; the heating can also be modified by acting on the power emitted by the wire heaters.
This glass temperature control process has a number of serious drawbacks especially when there is a ribbon of transparent glass and/or of glass with a highly reflecting coating on one face.
As a matter of fact, glass is a poor conductor of heat and during its cooling a noticeable temperature gradient appears between the surface and the middle part of the glass ribbon. If the glass is still plastic, the stresses created by the temperature gradient produce a deformation of the glass ribbon. If the glass is sufficiently cooled and perfectly elastic the temperature gradient creates internal stresses in the glass ribbon which remain since the ribbon is no longer plastic.
Experience shows that, in order to reduce as much as possible the effect of the temperature gradient, it is advisable to achieve an even glass cooling over the whole glass lehr length, i.e. the glass temperature should decrease approx. in a linear way. Now, it should be noted that when cold air is introduced at the inlet end of the exchanging tubes, the cooling is not at all even; glass is cooled much more rapidly at the outlet than at the inlet of the lehr zone concerned so that internal stresses are induced in glass in the middle of the annealing range.
On the other hand, glass is cooled more rapidly on the longitudinal edges than in the middle part. If, in order to remedy this transverse temperature gradient, the heating means located close to the longitudinal edges of the float glass lehr are put into operation, these heating means radiate on the adjacent cooling means; owing to this interaction it is difficult to achieve an optimum and uniform temperature over the whole width of the glass ribbon.
It is very difficult to remedy these drawbacks as the cooling means have a very long response time whereas the electrical heating means have a relatively short response time and can cause glass overheating when they are brought into operation.